Over 30% of online ad impressions aren't seen

"To better understand the quality of ad delivery today, comScore conducted a U.S.-based vCE Charter Study, which involved 12 national brands, 3,000 placements, 381,000 site domains and 1.7 billion ad impressions. Select advertisers from the charter study include Allstate, Chrysler, Discover, E*TRADE Financial, Ford, General Mills, Kellogg’s, Kimberly Clark, Kraft, and Sprint. All of the impressions analyzed in the study were delivered in iframes and none required publisher site pixels.
Topline study results are included below, and a full whitepaper with detailed insights and analysis will be published in March 2012.
Across all charter campaigns measured, 69 percent of the ad impressions were classified as being ‘in-view.’* The remaining 31 percent were delivered but never seen by a consumer, a likely result of a consumer scrolling past the ad before it loaded or a consumer never scrolling the ad into view. In-view percentages varied by site and ranged from 7 percent to 91 percent.
An average of 4 percent of ad impressions were delivered outside the desired geography, but individual campaigns ran as high as 15 percent. In many cases, ads were served in markets where the advertised product is not sold, meaning wasted ad spend and sub-optimal effectiveness results.
72 percent of Charter Study campaigns had at least some ads running next to content deemed “not brand safe” by the advertiser, meaning that the content is deemed objectionable by the brand. This type of unsafe delivery has the potential to damage the brand, creating a difficult situation for all members of the digital advertising ecosystem."

Source:  Press release from comScore, 18th January 2012

Note:  While this research is valid, it's worth mentioning the different ad trading models.  Most ads are not bought on a 'cost per view' (CPM) model, but are bought on a cost per click (CPC) or cost per action (CPA) basis.  This means that the advertisers for these campaigns don't pay for the ads until there is a click or a specified action - like a sale or a registration - so they are not being charge anything for ads that aren't seen. 

Part of the job of the agency is to ensure that the ads bought on a CPM basis appear in good positions - that is 'in view' making the site push the other ads down to the bottom of the page.  Similarly with ads appearing in unsafe content - part of the job of the agency is to make sure that this does not happen.
Also, it's occurred to me that the wording talks about ads, so a small ad has just as much weight in the study as a large ad.  It's far more likely that small ads (e.g. buttons 60 pixels x 60 pixels) won't be seen rather than big 790 pixel wide mastheads, so why not give the figures as % of ad real estate?